Piezoceramic actuators are used and optimized for a large number of applications in virtually all fields of technology. The application of an electrical field to the piezoceramic actuator results in a geometrical distortion, particularly in a change of length and height. In order to reduce the voltage to be applied to the actuator and/or increase its distortion, piezoceramic actuators usually comprise a large number of thin piezoceramic layers and internal electrodes arranged alternatingly in a stack.
During manufacturing, the piezoceramic layers are provided as green sheets and laminated with the internal electrodes to form a stack. The stack is burned or heated to a sintering temperature of the piezoceramic layers thereby transferring the sheets from the green state to the ceramic state. External electrodes are deposited on or arranged at lateral surfaces of the stack and connected to the internal electrodes in an electrically conductive way. Each internal electrode is connected to only one of the (at least) two external electrodes. This is achieved by providing the internal electrodes in such a way that the edge of an internal electrode which is to be connected to an external electrode on the lateral surface is flush with this lateral surface while there is provided a distance between the edge of an internal electrode and a lateral surface if the internal electrode is to be electrically insulated from an external electrode on this lateral surface.
Particularly due to temperature gradients during the cooling phase after the sintering process and due to lateral inhomogeneities within the piezoceramic layers (e.g. microstructural differences), mechanical stress within the stack arises.
After baking the stack of piezoceramic layers, a high electrical field is applied to the piezoceramic layers by applying a corresponding electrical voltage to the internal electrodes via the external electrodes. The value of the electrical field is selected such that a permanent electrical polarization and a permanent distortion of the piezoceramic layers remains after switching off the electrical field. This procedure is called poling.
Since the internal electrodes do not extend to all lateral surfaces of the stack, the piezoceramic layers are not exposed to a laterally homogeneous electrical field and are not polarized and distorted in a laterally homogeneous way. Rather, close to the lateral surfaces, mechanical stress inside the piezoceramic layers results. This mechanical stress is further increased during operation of the piezoceramic actuator. Cracks occur in the stack both inside the piezoceramic layers and along the internal electrodes and frequently cause a failure of the piezoceramic actuator.
According to EP 0 479 328 A2, slits in a piezoelectric actuator reduce tensile stress.
According to DE 103 07 825 A1, a ceramic rupture layer is arranged between two ceramic layers of a piezoceramic multilayer actuator, the ceramic rupture layer forming a predetermined breaking layer.
According to DE 102 34 787 C1, microdisturbances are incorporated in the piezoceramic layers of an actuator in such a manner that these act as crack sources located quasi at a pre-known site, with the crack growth being controllable.
According to DE 10 2004 031 402 A1, a piezoelectric device comprises a predetermined breaking point in an internal electrode.